Refer to FIG. 1 for a sensing unit 1 of a conventional gas flow meter. The sensing unit 1 is disposed in an accommodation space 20 of a tube 2 and used to measure the gas flow rate in an inner space 30 of a measured pipe 3. The sensing unit 1 includes a base 11 connected with one end of the tube 2, which is near the inner space 30 of the measured pipe 3; a speed transducing element 12 penetrating the base 11; and a temperature transducing element 13 penetrating the base 11. Two ends of the speed transducing element 12 are respectively disposed in the accommodation space 20 and the inner space 30 of the pipe 3. Two ends of the temperature transducing element 13 are respectively disposed in the accommodation space 20 and the inner space 30 of the pipe 3.
The temperature transducing element 13 is a multifunctional element including different control modes respectively to detect the ambient temperature and compensate for temperature variation. The internal processing unit of the temperature transducing element 13 has to switch the control mode to determine whether to detect the ambient temperature or compensate for the temperature drop of the speed transducing element 12. As the internal processing unit must spend time in switching the control mode, the temperature transducing element 13 cannot compensate for the temperature drop of the speed transducing element 12 instantly, which is the primary cause of errors.
The second cause of errors is that the microcontroller does not provide the compensated value flexibly according to the actual temperature variation. In general, a single compensated value is sufficient for the gases of the same composition and the gases with slight temperature variation.
For the gases of different compositions and the gases with greater temperature variation, a single compensated value set in the microcontroller is only available for the temperature variation between 0-30° C. Once the temperature variation is greater than 30° C., the single compensated value would cause errors in flow rate statistics. According to the results of practical statistics, the greater the temperature variation, the larger the statistic error value. Therefore, the conventional sensing unit still has room to improve.